Lincoln thompson



- unless Patented Jan. 27, 193

UNITED STATES PATENT OFFICE TALKING MACHINE CQMPANY, A CORPORATION OF NEW JERSEY METHOD OF ACTIV ATING II'LAMEN'IB No Drawing. I

This invention relates to a method for activating filaments and more particularly to a method or process for increasing the electron discharge of filaments forevacuated vessels of the type employed in electrical systems for the transmission and reception of signals.

It has been found that highlyheated filaments or electrodes, such as platinum filaments, when coated with certain metals or metallic compounds, as oxides of. barium, strontium or calcium, .for example, emit far more negative ions or electrons per square centimeter per second than filaments formed of pure platinum at the same temperature. Other substances have been similarly emplayed with satisfactory results.

It has heretofore been proposed to coat filaments with an activating substance by what may be termed the resln process, the essential steps of which consist in immersing the filament a number of times in a molten mixture containing the activating substance and resin, and, after each immersion, heating the filament to a sufiiciently high temper-- ature to burn oil the resin. Finall the coated filament is heated for a num er of hours at a temperature of approximately 1200 C. to remove all volatile substance. This process is unsatisfactory due to the fact that a considerable amount of time, approximately ten hours, is required for'carrying out the same. About fifteen immersions are required to secure the desired coating, and the final heating usually extends over a period of approximately eight hours. Much skill is requlred to secure the proper viscosity of the resin mixture, in volatllizmg the resin after each immersion, and in handling the filament so as not to remove the delicate coating. Moreover, a resin treated filament, when in use in a vacuum tube, will-emit gas great care is taken to thoroug 1y bake it.

It has also been proposedto mechanically Application filed April 1, 1925. Serial No. 20,009.

material bein non-ductile, and the mixture must be extru ed to form the filament, which, when formed, has practically no tensile strength andfis easily broken.

An object of the present invention is to provide a method for activating filaments which includes the use of amalgamation as a means for impregnating or coating the filaments with the activating substance.

Another object of the present invention is to provide a novel method of the character described whereby the activating substance not only forms an outer coating but is preferably incorporated into the cross-section of the filament, thus increasing the length of tiring that the filament may be satisfactorily us A further object of this invention is to rovide a method of the above character w 'ch may be successfully employed for activating filaments that are reformed of a base substance that is pref rably ductile.

Other objects relate to the provision of a method which is simple and re id in operation, which is economical and e cient, which does not require highly skilled o eratives for carrying out the same, and whic is productive of a strong, long-lived filament of high electron discharge.

Broadly, the invention includes a method for activating filaments wherein a filament formed of any suitable material, such as latinum, is subjected to mercury or an ama gam and an activating substance is incorporate into or coated onto the filament by amalgamation after which the mercury is driven ofi.

The invention is capable of being carried out in a variety of ways, some of which have been described in articular detail herein but it is to be express y understood that the invention is not limited thereto since various changes may be made in the character and order of steps within the scope of the up;

pended claims.

The filament to be treated, which is prefers;

bly platinum wire, is suitably attached or supported in an suitable manner on a wire frame. If pre erred, this frame, may be formed of nickel wire and rovided with hooks of the same material or supporting 1 taken to -thoroughly clean the filament and prises the anode,

have a few cubic centimeters of free mercury g frame prior to subjecting the filament to the activating process. This cleaning may be satisfactorily done by immersing the filament and frame in warm alcohol, after which the parts are dried and the frame passed quickly over a gas flame to render the platinum incandescent, thereby further cleaning it.

The activation of the filament by amalgamation may be effected in a variety of ways in accordance with the procedure which I now consider preferable, the filament is first immersed in a hot amalgamating solution, which solution preferably consists of abath of clean mercury, to which has been added a few small chips of pure zinc to facilitate the amalgamation. On top of the mercury bath is a layer of dilute hydrochloric acid, about one part of concentrated acid to ten partsof water have been found to be suitable. The filament is alternately immersed a number of times in the mercury bath and the acid layer, and when the filament will sink under the mercury layer, the amalgamation is completed. If the filament is now examined under a magnifying glass, it will be found covered with a convex film of mercury over its entire surface.

In order to secure the most satisfactory results, the zinc should be renewed each time the bath is used and the-acid will also have to be fre uently renewed. The mercury should be requently cleaned and dried by filtering it through a paper cone with a fine hole in it.

The next 'step in the process consists in washin the filament in cold water and then subjecting the same to electrolysis. Preferably, a saturated electrolytic solution is employed consisting of equal volumes of saturated solutions of salts of alkaline earth metals, such as barium and strontium chlorides. The amalgamated filament and frame is placed in this solution and constitutes the cathode, while a sheet of pure platinum of approximately the area of the cathode, comthe latter being preferably positioned a distance of'approximately two centimeters from the cathode. A current is then passed through the electrolyte and it has been found that satisfactory results are obtained by employing a current density of approximately .1 of an ampere per square centimeter of cathode surface. It is advisable to in the bath to add to the cathode area and to aid'in the building up of the amalgam. rapid stream of chlorine evolves from the anode and after a period of one-half to threequarters of an hour, tiny crystals of barium and strontium amalgam will appear on the filament. Sometimes these crystals build up to large agglomerates about the filament and support, and care must be taken to prevent too great an accumulation ofthese crystals as the weight of the same may result in the breakage of the filament.

The filament is now removed from the electrolyte, quickly washed in cold water to remove chlorides, and together with its frame,

is placed in a furnace maintained at a suitable temperature, approximately 800 C. having been found to be satisfactory. Prefermersed in an amalgam containing alkaline earth metals, such as barium and strontium, in about equal amounts.

The amalgams of barium and strontium may be easily made by the electrolysis of saturated solutions of the chlorides of these substances, preferably a platinum anode, a mercury cathode, and a current density of about 1/15 of an ampere per square centimeter of cathode surface are employed. The electrolysis is carried on until the mercury cathode is full of crystals of the amalgam showing super-saturation of the mercury with the alkaline earth metal. As these amalgams are readily oxidized in air they should be protected from the air as by a layer of kerosene. The amalgam is then diluted to a point where it is slightly fluid, giving a solution of approximately of 1% of the alkaline earth metal. The exact proportions are not critical but the amalgam should be as concentrated as possible .without having any great quantity of solid amalgam separating from it. The separately formed saturated solutions of barium and strontium amalgams are then mixed and used for the immersion of the filament.

The previously amalgamated filament is immersed in this bath, preferably for at least one hour, to secure thorough impregnation. The composite amalgam bath should be covered with a layer of kerosene during the immersion of the filament therein or said bath should be used under an atmosphere of hydroen. The impregnated filament is then heated as before, to drive off the mercury,

A whereupon it is ready for use.

Other substances than those mentioned above may be employed for activating filaments. For example uranium, thorium, yttrium, or any suitable metal that can be amalgamated is available. When uranium is used as the activating substance the filament is amalgamated by electrolysis,in amannor similar to that previously described. The electrolyte employed in this instance, however, is a saturated uranium chloride solution. The platinum anode employed is preferably enclosed ina porous cup to keep the chlorine away from the deposited uramum. approximately .01 per ampere per square centimeter and the time of the run should be approximately three hours. After the amalgamation the filament is hung in a tube, preferably of pyrex, through which hydrogen is continually passed and the'filament subjected to a heat of about 800 C. for approximately one hour to drive off the mercury. This lament should be kept away from the air until used to avoid oxidation of the uranium.

A. variety of thermionically active salts may be employed as an activating substance in place of the substances heretofore mentioned. For example, the method may be carried out exactly-as described in connection with the use of uranium to the point where the mercury is to be distilled off. The

heating of the filament is done, however, in

an atmosphere of chlorine instead of hydrogen which results in the production of uranium chloride on and Within the filament.

The above described methods'have related to the use of a filament formed of platinum. Filament cores other than platinum may be satisfactorily employed, however. For example, if it is desired to use iron for the filament core, the iron filament must be carefully cleanedin dilute sulphuric acid and then immersed in an amalgamating solution.- Preferably, the latter is composed of twelve parts mercury, one part zinc, two parts ferrous sulphate, one and one-half parts of concentrated hydrochloric acid and twelve parts of water. This amalgamating solution should be used boiling hot. After the cleaning and immersion'steps, the alkaline earth amalgam is applied as previously described. Distillation of the mercury, however, from the filament is preferably carried out in hydrogen or inert gas to prevent oxidation of the iron. Oxidation of the earth metal is obtained by exposing the treated filament for several hours in the air. Uranium or other activating metals or salts may be applied to iron cores if similar precautions are taken. It will be noted that the process does not have to be carriedout in a vacuum and mercury is preferably drivenfrom the filament in air so as to simultaneously form the oxide, although if desired the driving off of the mercury and the formation of the oxide may be separately performed. If an lllltlGSlitlhlG compound should be formed, however, during the driving ofi of the mercury in a gas-.

eons atmosphere, as for example. if a hydride were formed under some conditions, a vac- The current density used should be.

given a plurality of treatments in any of-the manners heretofore described should it be desired to increase the thickness of the coating; for example, a filament'may beamalgamated with an alkaline earth 'metaL'the mercury driven off, and these steps repeated one or more times.

. It will therefore be perceived that a process described with cons derable particularity, it'

is to be expressly understood that the invention is not limited thereto for various changes may be made in the order and character of the steps of the process without departing from the spirit of the'invention. For example, the mercury and the activating substance might be simultaneously electro-deposited; other means than electro-deposition might be employed for introducing the activating substance into the amalgamated filament, such as contact with the activating metal, mechanical rubbing, etc; and the final removal of the mercury might be accomplished by solution in acid, or other means than boiling. In fact the present invention provides for the activation of filaments of any material capable of amalgamation with mercury with any suitable activating metal, alloy or salts or mixtures of salts which also are capable of amalgamation with mercury.

Reference is therefore to be had to the claims hereto appended for a definition of the limits of the invention.

What is claimed is:

1. A process for activating filaments which includes amalgamating the filament. treating the filament with one or more activating substances, and heating the filament to drive off mercury.

2. A process for activating filaments which includes treating the filament with mercury, impregnating the filament with one'or more activating substances, and heating the filament to drive off the mercury.

3. A process for activating filaments which includes subjecting a filament to a bath of mercury, impregnating the filament with one or more activating substances, and heating the filament to drive off the mercury and oxidize the activating substance.

- 4. A process for activating filaments which includes immersing a filament in a bath containing mercuryand zinc, treating said filament with one or more activating substances,

and heating the filament to drive 05 all of the mercury.

5. A process for activating filaments which includes subjecting a filament to a bath of mercury and zinc, treating the filament'with an activating substance, and heating the filament to drive off all of the mercury and oxidize the activating substance.

6. A .process for activatingfilaments which includes amalgamating the filament, treating the filament with one or more activating sub stances, and subjecting the filament to a high temperature to simultaneously oxidize the activating substance and drive ofi the mercury.

7. A process for activatin filaments which includes amalgamating the lament, amalgamating therewith an activating substance, and heating the filament to drive off the mercury.

8. A process for activating filaments which includes treatin the filament with mercury, amalgamating therewith one or more activating substances, and heating the filament to oxidize the activating substance.

9. A process for activating filaments which includes treatin amalgamating tfierewith one or more activating substances, and heating the filament in an atmosphere containing oxygen to drive ofi the mercury and oxidize the activating substance.

simultaneously.

10. A process for activating filaments which includes forming a filament, amalgamating one or more activating substances therewith, and driving off the mercury.

11. A process for activating filaments which includes forming a filament, amalgamating one or more activating substances therewith. driving off the mercury, and heating said filament in an atmosphere to form an activating compound.

12. A process for activating. filaments which includes forming a filament, almalgamating an activating substance therewith, and heating said filament in an atmosphere containing oxygen to drive ofi the mercury and oxidize the activating substance.

13. A process for activating filaments which includes forming a filament of ductile metal, amalgamating an activating substance therewith, and driving off the mercury.

14. A process for activating filaments which includes forming a filament of ductile metal, amalgamating an activating substance therewith, and heating the filament in. an atmosphere to form an activating compound therewith.

15. A process for activating filaments which includes amalgamating a platinum filament with one or more activating substances and driving off the mercury.

16. A process for activating filaments which includes treating a latinum filament with mercury, amalgamating therewith one activatin the filament with mercury,

or more activating substances, and heating the filament to drive 01! the mercury.

17. A process for activating filaments which includes treating the filament with mercury, placing said filament in a solution of one or more activating substances, passing a current through said solution to deposit the substance on said filament, and heating t e filament to oxidize said activating substance.

18. A process for activating filaments which includes alternately treating a filament to a mercury bath and an acid solution, depositing one or more activating substances thereon by electrolysis, and heating the filament to drive ofi the mercury and oxidize the activating substance.

19. A process for activating filaments which includes treatin the filament with mercury, placing said lament in a solution of one or more activating substances, passing a current through said solution to deposit the activating substance on said filament, and

, drivingofi the mercury from said filament.

20. process for activating filaments which includes treating the filament with mercury, placing said filament in a solution of one or more activating substances, passing a current through said solution to deposit the activating1 substance on said .filament, and heating t e filament in an atmosphere containin oxygen to drive ofi the mercury.

21. he process of activating a filament which includes amalgamating a filament, amalgamating one or more activating substances with said filament, heating said filament in an atmosphere of hydrogen, and exposing said filament to the air to oxidize the activating substance.

22. A process for activating filaments which includes amalgamating a filament, amalgamating one or more activating substances with said filament, heating said filament in a neutral atmosphere and subsequently oxidizing the activating substance.

23; A rocess for activatin filaments which inc udes amalgamating t e filament, immersing the filament in an amal am containing one or more activating su stances,

and heating the filament to drive ofi the mer- 2i. A recess for activating filaments which inc udes amalgamating the filament, immersing the filament in an amal am containing one or more activatin su stances, heating the filament to oxidize t e activating substance.

25. A process for activating filaments which includes amalgamating the filament, immersing the filament in an amalgam containing one or more activating substances, and heating the filament in an atmosphere containing oxygen to drive oil the mercury and oxidize the activating substances.

26. A process for activating filaments IIMOAAQv which includes forming' a filament amalgamating one or more activating substances therewith and thereafter heating said amalgamated filament in a neutral atmosphere to 5 drive ofi the mercury.

In testimony, whereof I have Signed this specification;

LINCOLN THOMPSQN. 

